Extractive distillation of aromatics



Jan 8, 1957 F. s. BoNDoR ET AL EXTRACTIVE DISTILLATION OF AROMATICS Filed June 25, 1955 United ice EXTRACTIVE DISTILLATION F AROMATICS Frank S. Bondor, Wilmington, Del., and Edward H. .Lebeis, Jr., Media, Pa., assignors to Hondry Process Corporation, Wilmington, Del., a corporation of Delaware Application June 25, 1953, Serial No. 364,221

-14 Claims. (Cl. 2in-39.5)

The present invention relates generally to the separation, concentration and/or puriication of aromatic hydrocarbons and is particularly concerned with improvements and vmethods for the recovery of mono-nuclear aromatic hydrocarbons such as benzene, toluene and xylenes .from their admixtures with non-aromatic hydrocarbon components, e. g., in a naphtha fraction.

`Hydrocarbon distillates boiling in the range-of gasoline and naphtha from various sources and particularly such distillates obtained b.y catalytic reforming of naphtha comprise aromatic hydrocarbons in admixture with naphthenes and parans having boiling points very close to those of the aromatic components of the distillate, which renders the separation of the aro-matic components by ordinary methods of fractional distillation extremely diicult or impossible. Moreover, certain of these components may form azeotropes with thearomatic hydrocarbon desired to 'be separated. Among the various sep* aration techniques which have been tried with varying degrees of success there are included: liquid solvent eX- traction, selective azeotropic distillation, selective chromatographic adsorption, andextractive distillation. The present invention is concerned with improvements in processes involving the last-named type of operation, which type of process olfers considerable flexibility and other advantages particularly from the standpoint of adaptability to different types of distillates as well as those of different boiling ranges.

The extraction of benzene from hydrocarbon fractions containing the lsame has been of increasing importance in recent years because of continued demands for ythis material for use as such or as an intermediate in numerous industrial processes. Also, with the increased production of reformed naphtha of desirably high aromatics content, the need for efficient methods for separation and recovery .of benzene, and of other individual mono-nuclear aromatic hydrocarbons, has also increased. The

improved method of the present invention is applicable to the recovery of benzene as well as to therecovery o toluene or xylenes (including ethyl benzene).

The more typical contaminants found associated: with benzene ina benzene cut derived from a catalytically reformed naphtha include methylcyclopentane, which is generally present as the predominating contaminating naphthene, and various paraflins, including n-hexane, Z-methyl-hexane, methylpentanes and dimethylpentanes. An agent having high selectivity in the separation by extractive distillation of methylcyclopentane from benzene as well as in the separation of usually occurring Cs and C7 paraflins from benzene would obviously be desirable for use in processes involving benzene recovery.

The extractive distillation operation above described is based on the principle that the extractiva distillation solvent effectively alters the activity coeicient of one or more components of the feed to render the same less or more volatile, and' since the relative volatility of the contaminatingvnaphthenes and parafns as .compared with y benzene (or methylated benzene) is greater in the presence of the solvent, a higher degree of separation is possible than that obtained by ordinary methods of -fractional distillation.

In the accompanying drawing the-flow sheet includes suiticient information to be self-explanatory.

In accordance with the present invention a mixture of aromatic and non-aromatic hydrocarbons, such as a closely cut naphthafraction containing one of the mononuclear aromaticsis subjected to extractive distillation in the presence of a butanediol or butylene glycol solvent, particularly one of the group comprising 2,3-butanediol and 1,3-butanediol, with the separation of a vapor overhead concentrated in non-aromatic contaminants and a liquid bottoms fraction containing the solvent concentrated with aromatic hydrocarbons.

ln a method of this type, it is desirable that the solvent employ-ed is higher boiling than the highest boiling comf ponentof the feed and that the boiling point of the solvent is lhigh enough to allow easy separation in the aromatics stripper and low-enough to permit reasonable reboiier temperatures. ln this and other respects the butylene ,glycols or butanediols have proved well suited, and this is particularly rthe case with those of the group comprising 2,3-butanediol and l,3butauediol, all the more as they are also commercially available. Both these compounds yboil higher than the highest boiling Xylene, ortho-Xylene (14d-.4 C), viz., at 182 C. 'and 204 C., respectively, so that the same solvent may be used for Xylenes separation as well as for benzene and toluene separation from non-aromatic hydrocarbons. Of these butanediols, the 2,3isomer iso-f primary importance, particularly as its boiling point, while high enough for the indicated purpose is considerably lower than that of the 1,3-isomer, whereby the use of correspondingly lower reboiler temperatures is made possible.

The miscibility of the said butanediol isomers4 with non-aromatic hydrocarbons, however, is limited, lparticularly with respect to methylcyclopentane and 2-4-dimethylpentane which are among the main contaminants of the aromatics to be recovered by the present process. This miscibility is still less for the 1,3-isomer than for the 2,'3-isomer, the activity coefcient of which, -moreover, is more favorable than that of the 1,3-isomer. On the other hand, in -an extractive distillation column, the concentration of nonsolvent in the single phase liquid on the ,trays should be as high as practical to give good throughput, but not so high as to reduce the relative volatility. An economic optimum value often used is about 30% non-solvent and 70% solvent. On the top tray of the extractive distillation section, the liquid would then ybe composed of solvent and 30% naphthenes and paraiiins. That high a concentration of naphthenes and paraflins, however, cannot be atttained with the use as solvents of one of the two isomeric butanediols here in question. For this reason `a secondary solvent or solubilizer is added in accordance with the present invention for increasing the solubility of the naphthenes and paramns.

A small amount of this solubilizer is generally suicient, while a greater amount thereof Will not be harmful if the added solubilizer also improves the relative volatilities of non-aromati'cs to aromatics. The system canV be operated in continuous ymanner by charging the solvent to the upper part of a fractional distillation tower and bringing in the hydrocarbon feed to be separated or puri-` ied near the middle of the tower. The resulting liquid fraction comprising the solvent and aromatics is fed to a stripper for separation and the solvent stripped of aromatics recirculated to the extraction tower.

While` numerous solvents are suitable to a lesser or greater degree for this purpose, the greatest increase in solubility has been obtained with the addition of octyl alcohol, particularly 2-ethyl hexanol, and with butyl Cellosolve (ethylene glycol monobutyl ether). Of these compounds the 2-ethyl hexanol has proved most satisfactory and is at present available at high purity and relatively low price. Thus, 44% of 2,4-dimethylpentane was dissolved in a mixture of 30% of 2ethyl hexanol and 70% of 2,3-butanediol at 75 C.

The use of the Z-ethyl hexanol as solubilizer is very advantageous also because its boiling point (183.5 C.) is very near to that of the 2,3-butanediol (182 C.), and because both boiling points are higher than that of the highest boiling component of the mixtures to be separated, viz., xylene. This proximity of the boiling points of both solvent and solubilizer also makes possible the recovery of their mixture for direct reuse upon their common separation on stripping the aromatic hydrocarbons from this solvent mixture. The activity coefficient and relative volatility improvement accorded to the nonaromatic hydrocarbons by the 2,3-butanediol, when used as the solvent, is not reduced to a great extent by the addition of 2-ethyl hexanol if the concentration thereof in the mixed solvents or mixture of solvent and solubilizer is not too high.

A mixture of about 7585% by volume of 2,3-butancdiol and 25l5% by volume of 2-ethyl hexanol has proved satisfactory in every respect. Thus, in the extractive fractional distillation of a mixture of benzene and methylcyclopentane (MCP), which is one of the most diicult of all hydrocarbon mixtures to be separated, the

following results were obtained:

Mole per- Relative Position in Column cont MGP Volatility. Boiling in Liquid M' CP/ Point, C. Benzene ln the use of pure phenol as the solvent instead of the said mixture of 2,3butanediol and Z-ethyl hexanol, under the same operating conditions, less favorable results, with respect to the relative volatilities of methylcyclopentane and benzene, could be obtained, and the mixture of butanediol and ethyl hexanol shows definite performance advantages over phenol in separating aromatic from nonaromatics.

Thus, while the butanediols, particularly the 1,3-butanediol and the 2,3-butanediol, and particularly the latter, have proved useful in the extractive distillation of mixtures of aromatic with non-aromatic hydrocarbons to separate them from one another, the solubility of parans in these solvents is not very high, and it is therefore proposed by this invention to add a secondary solvent or solubilizer, whereby the undesirable formation of two liquid phases in the extractiva distillation section of the distilling column is avoided. Of the octyl alcohols mainly to be considered as solubilizing additives for this purpose, the 2-ethyl hexanol has proved particularly useful, as its boiling point is very close to that of the 2,3-butanediol and as it distinctly increases the solubility of parains, which are normally least soluble, in the 2,3-butanedoil even when only small additions of the alcohol are made. Thus, a mixture of 75-85 parts of 2,3-butanedio1 and 25-15 parts of 2-ethyl hexanol improves the relative volatility of the aromatic/non-aromatic system to be treated very considerably and only slightly less than is obtained in the use of the 2,3-butanediol alone. Such a mixture, therefore, is very useful for this purpose. The boiling points of both the primary and secondary solvents, while higher than the boiling point of the highest boiling xylene to be considered, are low enough to allow easy redistillation for purifying purposes.

In order to simplify the separation process, it is advisable though not indispensable, to make use of a close fraction of the feed stocks for the extractive distillation proper, thus making possible a larger throughput or the use of a smaller extractive distillation apparatus. Thus, naphtha to be subjected to an extractive distillation in accordance with the present invention, is preliminarily subjected to prefractionation separating out respectively a benzene cut and, if desired, a toluene cut of about 10 F. spread in boiling range.

Any type of naphtha or hydrocarbon mixtures containing aromatics and non-aromatics may be treated by the new process, such as cracked naphtha as obtained in the catalytic or pyrolytic conversion of mineral oils, reformed naphtha, polyformed naphtha, straight run naphthas or gasolines containing aromatics, and so on.

Obviously, many modifications and variations of the invention as hereinbefore set forth may be made without,

departing from the spirit and scope thereof, and, therefore, only such limitations should be imposed as are indicated in the appended claims.

What is claimed is:

l. The method of separating aromatic hydrocarbons from mixtures containing the same and non-aromatic hydrocarbons which comprises subjecting such a mixture to extractive distillation in the presence of a butanediol with addition of octyl alcohol as a solubilizer for increasing the solubility of the non-aromatic hydrocarbons in the butanediol, recovering the liquid fraction comprising said butanediol solvent and said octy alcohol solubilizer containing extracted aromatics, and stripping the aromatics from said solvent and solubilizer.

2. The method of separating aromatic hydrocarbons from mixtures containing the same and non-aromatic hydrocarbons which comprises subjecting such a mixture to extractive distillation in the presence of 2,3-butanediol solvent and 2-ethyl hexanol solubilizer, recovering the liquid fraction comprising said solvent, said solubilizer and extracted aromatics, and stripping the aromatics from the mixture of solvent and solubilizer.

3. The method according to claim 2 wherein the mixture of said solvent and said solubilizer stripped of aromatics is recycled to the extractive distillation step.

4. The method of separating aromatic hydrocarbons of the group comprising benzene. toluene, and xylenes from mixtures containing the same and non-aromatics in accordance with claim 2.

5. The method of separating aromatic hydrocarbons of the group comprising benzene, toluene and xylenes from mixtures containing them together witth naphthenes and parans, in accordance with claim 2.

6. The method according to claim 2 wherein the amount of said solvent used is substantially in excess of the amount of said solubilizer.

7. The method according to claim 6 wherein a mixture of 75-85% by volume of 2,3-butanediol and of 25-l5% by volume of 2-ethyl hexanol is employed.

8. The method of separating aromatic hydrocarbons from aromatic-bearing naphthas which comprises prefractionating said aromatic-bearing naphthas for concentrating the contents thereof in aromatics, subjecting the aromatic-rich naphtha fraction thus obtained to extractive distillation in the presence of 2,3-butanediol solvent and 2-ethyl hexanol solubilizer, recovering the liquid fraction comprising said solvent, said solubilizer and extracted aromatics, and stripping the aromatics from the mixture of solvent and solubilizer.

9. The method which comprises feeding aromatics containing naphtha to an intermediate portion of a fractional distillation tower while a mixture of 2,3-butanediol and 2-ethyl hexanol is charged to the upper part of said tower, withdrawing a vapor overhead rich in non-aromatic hydrocarbons and a bottoms fraction containing said mixtures concentrated in aromatics.

l0. The method according to claim 9 wherein said naphtha is a distillate of a catalytic reformate.

11. The method according to claim l0 wherein said distillate is a closely cut fraction comprising benzene.

12. The method according to claim 10 wherein said distillate is a closely cut fraction comprising toluene.

13. The method according to claim 10 wherein said distillate is a closely cut fraction comprising one or more of the Xylenes.

14. The method which comprises contacting a naphtha reformate fraction, of a boiling range incuding benzene, with solvent comprising 2,3-butanediol and 2-ethyl hexanol under temperature conditions to effect removal of a vapor overhead having a relative boiling range below that of said benzene and said solvent and the formation of liquid aromatic concentrate rich in benzene, separately distilling said concentrate to recover benzene, and returning the residue comprising said solvent to further contacting of naphtha reformate.

References Cited in the le of this patent UNITED STATES PATENTS Re. 22,379 Dunn et al. Sept. 28, 1943 1,919,752 Schmidt July 25, 1933 2,215,915 Cope et al. Sept. 24, 1940 2,325,379 Durrum July 27, 1943 2,370,530 Gage Feb. 27, 1945 2,373,951 Evans et al Apr. 17, 1945 2,415,192 Rittenhouse Feb. 4, 1947 2,460,852 Shiras et al. Feb. 8, 1949 2,496,253 Purcell et al. Jan. 31, 1950 2,567,228 Morrell et al Sept. 11, 1951 2,706,707 Morrell et al. Apr. 19, 1955 OTHER REFERENCES Smith et al.: Extraction of Aromatics from Gasoline, Chemical and Metallurgical Engineering, January, 1944. 

1. THE METHOD OF SEPARATING AROMATIC HYDROCARBONS FROM MIXTURES CONTAINING THE SAME AND NON-AROMATIC HYDROCARBONS WHICH COMPRISES SUBJECTING SUCH A MIXTURE TO EXTRACTIVE DISTILLATION IN THE PRESENCE OF A BUTANEDIOL WITH ADDITION OF OCTYL ALCOHOL AS A SOLUBILIZER FOR INCREASING THE SOLUBILITY OF THE NON-AROMATIC HYDROCARBONS IN THE BUTANEDIOL, RECOVERING THE LIQUID FRACTION COMPRISING SAID BUTANEDIOL SOLVENT AND SAID OCTY ALCOHOL SOLUBILIZER CONTAINING EXTRACTED AROMATICS, AND STRIPPING THE AROMATICS FROM SAID SOLVENT AND SOLUBILIZER. 